A popular option for aortic valve replacement is to retain the native aortic root and the normal coronary artery attachments and secure the replacement prosthesis inside the patient's own aorta. With this procedure, only the valve is replaced and not the entire root. It is unnecessary to re-attach the coronary arteries and, should repeat surgery be necessary, a surgeon must only replace the valve and not an entire section of the aorta. When a surgeon replaces the aortic valve in this manner, the patient is first placed on a heart-lung machine and the section of the aorta having the aortic valve is clamped off to allow access. That section of the aorta is therefore collapsed and unpressurized leaving a pressurized section connected to the heart-lung machine. The unpressurized section of aorta is then opened and the diseased valve is removed in its entirety, including careful removal of calcium deposits within the aorta and annulus. The aorta and sinotubular junction are then sized and the surgeon prepares the appropriate replacement valve. The surgeon then sutures the inflow or annular end of the replacement valve into the inside of the aorta. When these sutures are drawn tight, the valve is pulled inside the aorta when approximately 20 sutures are then applied around the annular end. The commisures of the replacement valve, which extend from the annular end, may or may not need to be affixed to the aorta as discussed below.
Two major types of prosthetic or replacement heart valves exist. The first general type of valve is a mechanical prosthesis which includes commisures that are self-supporting and do not need to be affixed to the aortic wall. Mechanical prostheses are generally formed entirely of artificial material, such as carbon fiber, titanium, dacron and teflon. While these mechanical prostheses are durable, relatively quick to implant and generally easy to manipulate during surgery, they also have certain disadvantages. For example, due to the artificial materials used in their construction, blood clots can form on the valve and subsequently cause valve failure. If the clot dislodges from the valve, the clot can lodge in a downstream vessel and cause stroke or organ ischemia. For these reasons, patients with mechanical heart valves must take anticoagulants for the rest of their lives. Anticoagulants bring about their own complications in some patients, including internal bleeding or other side effects.
The second major type of prosthetic or replacement heart valve is a biologic valve. This category includes valves harvested from human cadavers, i.e., allografts or homografts, or animal tissue generally harvested from cows and pigs. More recently, there has been increasing effort to develop synthetic biologically compatible materials to substitute for these natural tissues. Among their advantages, biologic prostheses generally do not require lifelong anticoagulation as they do not often lead to clot formation. These valves are provided in stented or unstented forms. A stented valve includes a permanent, rigid frame for supporting the valve, including the commisures, during and after implantation. The frames can take the form of a wire or other metal framework or a plastic frame encased within a flexible fabric covering. Unstented valves do not have built-in commisure support so surgeons must use their skill and best judgement to determine the optimal site of implantation inside the patient's native aorta to maintain valve competence. When securing the valve commisures, obstruction of the patient's native coronary arteries must be avoided or myocardial infarction may result.
There are many limitations to procedures utilizing permanently stented biologic replacement valves. First, allografts (human cadaver donor valves) are not generally available with permanent frames or stents. Second, the frames or stents can take up valuable space inside the aorta such that there is a narrowing at the site of valve implantation. This narrowing leads to pressure gradients and increased loads on the left ventricle and, therefore, increased incidence of hypertrophy and reduced patient survival. The frame includes artificial materials which can increase the risk of new infection or perpetuate an existing infection. It is also very important to realize that although the permanent frames or stents guarantee alignment of the commisures, they cause very high stresses on the commisures when the valve cusps move between open and closed positions. A patient's natural commisures are not placed under significant strain during opening and closing of the valve due to the natural resilience of the aorta. On the other hand, artificially mounted valves place the commisures under strain during operation of the valve due to the rigid materials of the frame. Over time, the valve cusps tend to decay under this strain and manifest calcification and tears which can lead to valve failure.
In many situations, biologic replacement heart valves are preferred in the unstented form due to the drawbacks mentioned above. Such valves are more resistant to infection when implanted free of any foreign material attachments, such as stents or frames. Also, the heart valve is more efficient when used without a stent. Efficiency refers to the pressure gradient across the valve during use. Natural human valves have almost no pressure gradient. When a natural heart valve is replaced by a biologic heart valve with a low pressure gradient, complications such as hypertrophy arise less often and result in improved patient survival.
Despite the known advantages of using biologic prosthetic heart valves without artificial supporting devices such as permanent stents or frames, relatively few surgeons employ this surgical technique due to its high level of difficulty. When unsupported or unstented by artificial devices, such as permanent stents, biologic replacement heart valves have a flimsy, soft and flexible nature. That is, the commisures of the heart valve do not support themselves in the proper orientation for implantation. For these reasons, it is very difficult to secure the commisures properly into place. In this regard, the surgeon must generally suture the individual commisures of the heart valve in exactly the proper orientation to allow the valve to fully and properly function.
During valve replacement surgery, an L-shaped retractor is placed inside the aorta to pull it open for access purposes. While this provides exposure, it distorts the aorta and may give the surgeon an incorrect impression of the correct valve position. Next, and especially with regard to unstented biologic valve procedures, the surgeon must guarantee that the commisures pass straight up the aorta at roughly right angles to the plane of the annulus. There is very little technology to help the surgeon correctly place the stentless replacement valve. To help confirm that the leaflets are correctly spaced at 120° apart, surgeons may use a disc having markings 120° apart. The surgeon can use this to roughly estimate the spacing by placing it near the distal ends of the commisures. However, this provides only a rough guide. For example, it is possible to equally space the commisures at the upper end and still have a valve placed in a skewed position. Finally, the aorta is not a straight tube at the surgical site, but instead flares outward at the surgical site. The valve must conform to the flare of the aorta at this location. Once the surgeon has completed an inspection for these three elements, i.e., correct spacing at approximately 120° between the commisures, correct perpendicular position of the commisures relative to the annulus plane, and appropriate conformation to the flare of the aorta, the surgeon must suture the commisures to the wall of the aorta. As this is done, it is necessary to make sure there is no encroachment on the ostia or origins of the coronary arteries. After the valve commisures are attached to the aorta and proper orientation and positioning is confirmed, the surgeon closes the aorta.
Following surgery, there is a risk that the aorta will dilate at the sinotubular junction months or years later and draw the valve commisures and attached cusps apart from each other. This will cause insufficiency and failure due to leakage through the valve. There is a further need for methods to ensure that late enlargement of the sinotubular junction does not necessitate reoperation for late valve insufficiency and failure.
In general, there is an increasing need for devices which improve the efficiency and reliability of implanting replacement heart valves. In conjunction with this, there is a need to improve these procedures so that all surgeons, not just those with the highest skill levels, can implant heart valves with superior results.